Refrigeration control system with relative humidity control



REFRIGERATION CONTROL SYSTEM WITH RELATIVE HUMIDITY CONTROL Filed July 23, 1938 \OO l0! ((MBMMUBOWM attorney Patented Oct. 7, 1941 REFRIGERATION CONTROL SYSTEM WITH 1 RELATIVE HUMIDITY CONTROL Alwin B. Newton, Minneapolis, Minn., assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware Application July 22 1938, Serial No. 220,889

2 Claims.

This invention relates to refrigeration control systems in general and more particularly to a refrigeration control system provided with a relative humidity control.

In conventional refrigerating .systems having a fin type evaporator for cooling 'a fixture or space such as a walk-in box, the relative humidity of the air therein is decreased below the desired value when desired dry bulb temperatures are maintained. In such a system the compressor may be controlled by a thermostat responsive to the fixture temperature and under normal load conditions with the thermostat set at, say 38 to 40, the relative humidity within the fixture will never exceed 70% to 75% which value is too low for the satisfactory storing of perishable articles such as meat, a drying action being present. This dehumidifying action in a conventional refrigerating system is caused by the accumulation of relatively large amounts of frost on the fin type evaporator which decreases the effective cooling area thereof by filling up the space between the fins with frost. By reason of this decrease in effective cooling area the surface of the frost must be maintained at a relatively lower temperature to maintain the desired dry bulb'temperature conditions within the fixture. This lower temperature of the surface of the frost which is necessary to perform the requisite cooling action condenses out moisture from the air to decrease the relative humidity thereof. This action is accumulative in that as the frost builds up the effective cooling area thereof further decreases which requires. a lower temperature to maintain the desired dry bulb conditions which in turn increases the dehumidifying action thereof.

With a dry bulbtemperature of, say 38 to ing the moisture condensed on the evaporator and the use of defrosting fans but these arrangements have not proved entirely satisfactory and are necessarily expensive to operate.

Accordingly, the prime object of this invention is to provide an improved refrigeration control system which not only maintains desired dry bulb conditions within=a fixture but also maintains desired relative humidity conditions without the use of expansive auxiliary equipment.

In carrying out this object of this invention, it is contemplated to utilize, first, a refrigeration control system wherein the compressor is inter-' mittently controlled by a thermostat to maintain desired temperature conditions in the fixture and wherein defrosting of the evaporator is accomplished periodically to prevent the building up of frost on the evaporator whereby the evaporator operates at a relatively high temperature which decreases the dehumidifying action thereof to a minimum. Although this portion of the invention may be carried out in any number of ways, I propose utilizing a refrigeration control system of the type illustrated in application Serial No. 196,447, filed by Albert L. Judson and Carl G. Kronmiller' on March 17, 1938. This control arrangement when operated in conjunction with a fixture thermostat maintains desired temperatures within the fixture and also prevents frosting of the evaporator. By reason of this frost preventing action the effective cooling area of the evaporator remains large at all times whereupon it is not necessary to reduce substantially the temperature of the evaporator to maintain the temperature of the fixture within desired limits Hence as the evaporator is maintained at higher temperatures to perform the sensible cooling the dehumidifying action of the evaporator is reduced to a minimum. It is found that with this arrangement with the thermostat set between, say 38 and 40 that the relative humidity under normal load conditions assumes values between 80% and 85% which ordinarily is satisfactory but which under some circumstances may be too high for the satisfactory storing of some perishable articles.

This invention therefore comprehends, secondly, the use of a refrigeration control system outlined immediately above in combination with a control arrangement responsive to the relative humidity of the fixture for increasing a desired amount the dehumidifying action of this control system. In other words, this invention contemplates the provision of a means for reducing the relative humidity in the fixture to the desired value, illustratively to Although this second feature may be accomplished in any number of ways, it is contemplated to utilize an ad- .justable superheat expansion valve of the type tion must be decreased to maintain the dry bulb temperature conditions of the fixture within the desired limits and accordingly the dehumidii'ying action of the efiective portion of the evaporator is increased. Desired relative humidity conditions are therefore maintained within the fixface of the evaporator |l-.

ed by a relative humidity responsive controller generally designated at 24. The relative humidity responsive controller 24 adjusts the expansion valve H! to vary the superheat setting thereof which in turn varies the effective cooling sur- Since the compressor l3 operates with a constant displacement, the result is to vary the temperature of this effective ture at all times as well as desired dry bulb temspace within desired limits while also maintaining desired temperature conditions therein.

Other objects and advantages will become apparent to those skilled in the art upon reference to the accompanying specification, claims, and

' drawing;

For a more thorough understanding of the in- .vention, reference is made to the accompanying drawing in which is diagrammatically illustrated the "preferred form of this invention.

For purposes of illustration the fixture or space, the temperature and relative humidity of which is to be controlled, is shown to be a walkin box In. The walk-in box In is cooled by an evaporator which is shown for purposes of illustration to be a fin type evaporator Refrigerant is supplied to and withdrawn from the evaporator I I by means of refrigerating apparatus generally designated at l2. This refrigerating apparatus may comprise a compressor |3 operated by an electric motor |4. Refrigerant is compressed in the compressor '|3 and passes through a high pressure line l5 to a condenser l6 wherein it is condensed. The condensed refrigerant flows through a receiver l1, a liquid line I8, and an expansion valve generally designated at 9 to the evaporator Expanded refrigerant is withdrawn from the evaporator I I through a suc-v tion line 20 by the compressor I3. Accordingly, when the refrigerating apparatus I2 is placed in operation the evaporator causes cooling of the fixture I0.

A temperature responsive controller generally designated at responds to the temperature of the air within the fixture l0 and operates to control the refrigerating apparatus l2 to maintain the temperature within the fixture within 'predetermined limits. A unitary control arrangement generally designated at 23 which may be of the type shown and described in the above referred to Judson and Kronmiller application causes intermittent or periodic defrosting of the evaporator II to prevent the formation of frost thereon whereby the evaporator operates at a relatively high temperature to maintain at a minimum the amount of latent heat removal. The expansion valve generally designated at |9 controls the supply of liquid refrigerant to the evapo-- rator II and this expansion valve may be adjustcooling surface to vary the amount of latent heat removal in order to maintain the relative humidity in the fixture at the desired value.

For purposes of illustration in this application the unitary control arrangement 23 is shown to comprise a base 26 upon which is mounted a bellows 21. The bellows 21 is connected by a pipe 28 to the suction line 20 of the refrigerating apparatus. The bellows 21 operates a lever 29 fulcrumed on'a fulcrum member 30 against the action of a tension spring 3|. One end of the tension spring 3| is connected to the lever 29, and the other end is connected to a .nut 32 screwthreadedly mounted on a screw 33. By rotating the screw 33 the tension in the spring 3| may be varied to adjust the pressure setting of this portion of the control arrangement. The lever 29 carries an insulating pad which in turn carries contacts 35 and 38. Contact 35 is adapted to engage a contact member 36 carried by a terminal 31 and contact 38 is adapted to engage a contact member 39 carried by a terminal 40. concentrically located cams 4| and 42 engage the contact members 36 and. 39 to relatively adjust the same. Upon an increase in suction pressure the contact 35 first engages the contact member 36 and then the contact 38 engages the contact member 39. Upon a decrease in suction pressure, contact 38 first disengages contact member 39 and then contact 35 disengagescontact member 36. For purposes of illustration, it is assumed that contact 35 and contact member 36 engage and disengage at a suction pressure value of 15 pounds and that contact 38 and contact member 39 engage and disengage at a suction pressure value of 30 pounds.

Also mounted on the base 26 is a second bellows 45 connected by a pipe 46 to the high pressure line |5 of the refrigerating apparatus. The bellows 45 operatesa lever 41 pivoted on a fulcrum member 48 against the action of a tension spring 49. One end of the tension spring 49 is connected to the lever 41 and the other end is connected to a nut 50 screw-threadedly mounted on a screw 5|. By rotating the screw 5|, the tension in the spring 49 isv varied to adjust the pressure setting of this portion of the control arrangement. The lever 41 adjustably carries an abutment member 52 having abutments 53 and 54. The abutment 53 is adapted to engage a contact member 55 carried by a terminal 56, the contact member 55 cooperating with a contact 51. The abutment 54 is adapted to engage a contact member 58 carried by the terminal 40, the contact member 58 cooperating with a contact 59. For'purposes of illustration, it is assumed that upon an increase in high pressure the abutment 54 first engages the contact member 58 at pounds to cause contact member 58 to disengage contact 59 and the abutment 53 to engage contact member 55 to cause contact member 55 to disengage contact 51 at pounds. Upon a decrease in high pressure contact member 55 engages contact 51 at 185 pounds and contact member 58 engages contact 59 at 135 pounds.

The unitary control arrangement 23 also includes a relay or starter generally designated at 62 which may comprise an operating coil 63 for operating bridge members 64 and 61. When the operating coil 63 is energized, the bridge member 64 is moved into engagement with contacts 65 and 66 and the bridge member 61 is moved into engagement with contacts 68 and 69. When the operating coil 63 is deenergized, the bridge members 64 and 61 are moved out of engagement with respective contacts by means of springs, gravity, or other means, not shown.

The unitary control arrangement may also include an overload cutout generally designatedat 12 which may comprise a base 13, a terminal 14, and a resistance connected between the terminal 14 and the contact 65 for heating a thermostatic element upon the occurrence of an overload condition to separate contacts 16 and 11. Following an overload cutout condition, the contacts 16 and 11 may be manually reset by means of a reset lever 18. The unitary control arrangement is usually provided with a control terminal 8| connected by a conductor 82 to the contact 68 but in this form of the invention this control terminal is not utilized.

Line wires 85 and 86 leading from some source ofpower not shown are connected to contact 51 and to a power terminal 83, respectively. The compressor motor 14 is connected by wires 88 and 89 across the contact 66 and power terminal 83. The contact 59 is connected by a conductor 96 to the contact 69 which in turn is connected ,for operating a lever 91 against the action of an adjustable tension spring 98. Lever 9'1 operates a switch 99. For purposes of illustration, it is assumed that the switch 99 is closed upon an increase in temperature to 40 and is opened whenthe temperature decreases to 38.

Assume now that the pressure on the high pressure side of the refrigerating apparatus is below 135 pounds to cause contact members 55 and 58 to engage their respective contacts 51 and 59, that the evaporator II has defrosted to cause the suction pressure to rise to 30 pounds to move the contacts 35 and 38 into engagement with their respective contact members 36 and 39, and that the temperature of the walk-in box has risen to 40 to close the switch 99. A starting circuit is thereupon completed for the relay 62 which may be traced from line wire 85 through contact 51, contact member 55, terminal 56, wire I06, switch 99, wire l8l, terminal 31, contact member 36, contacts 35 and 38, contact members 39 and 56, contact 59, conductor 96, contact 69, conductor 9|, contacts '16 and '11, conductor 92, operating coil 63, conductor 93, and power terminal 83 back to the line wire 86. Completion of this circuit energizes the operating coil 63 to move the bridge members 64 and 61 into en- The contact 11 is connected by terminal83 back to the other line Wire 86. Accordingly, when the relay or starter 62 is pulled in the compressor motor I4 is placed in operation.

Movement of the bridge member 61 into engagement with the contacts 68 and 69 completes a maintaining circuit for the relay or starter which is independent of the contact members 39 and 58 and which may betraced from the line wire 85 through contact 51, contact member 55, terminal 56, wire I09, switch 99, wire IM, terminal 31, contact member 36, contact 35, conductor 94, contact 68, bridge member 61, contact 69, conductor 9|, contacts '16 and 11, conductor 92, operating coil 63, conductor 93, and power terminal 83 back to the other line wire 86. Accordingly, the refrigerating apparatus is maintained in operation until either the temperature of the fixture l6 decreases to 38 or the suction pressure decreases to 15 pounds or the high pressure increases to 185 pounds. When any of these contingencies occur, the relay 62 is dropped out to stop operation of the refrigerating apparatus and the refrigerating apparatus cannot again be placed in operation until the temperature controller 22 calls for cooling, the high pressure decreases to 135 pounds, and the suction pressure rises to pounds.

For purposes of illustration, it, is assumed that a 30 pound suction pressure can only occur after the evaporator II has defrosted and therefore the refrigerating apparatus cannot be placed in operation until the evaporator has defrosted. The switching mechanism of the unitary control arrangement operated by the suction pressure bellows 21 therefore is operated in accordance with a condition indicative of evaporator temperature. Since the refrigerating apparatus cannot be placed in operation until the evaporator II has defrosted, the continued building up of frost on the evaporator is prevented and therefore the evaporator will perform its cooling function while operating at a relatively high temperature which decreases the amount of moisture taken out of the air within the fixture III. In other words, by utilizing the unitary control arrangement 23 in conjunction with the temperature responsive controller 22, desired temperatures are maintained within the fixture l0 and satisfactory for storing perishable articles but under some circumstances this relative humidity may be excessive wherein it is desirable to maintain the relative humidity between 75% and Since an excessive relative humidity may be obtained by the combined thermostatic and defrosting control arrangement outlined above, it is therefore possible to increase the amount of latent heat removal and control this increase in accordance with relative humidity to maintain any desired relative humidity within the fixture l6. Although this may be accomplished in any number of ways, I have shown for purposes of illustration an adjustable expansion valve of the constant superheat type controlled by the relative humidity controller 24. This expansion valve 19 may be of the type shown and described in the above referred to Newton application, and for purposes of illustration in this application it is shown to comprise a casing I having a chamber I08. Liquid refrigerant enters the chamber I06 through a passage I01 and discharges from the chamber I06 through an outlet valve seat I08. The discharge of liquid from the chamber I06 to the evaporator II is controlled by a valve I09 carried by a bell-crank lever IIO pivoted at III. The bell-crank lever is adjustably positioned by a pin H2 which in turn is carried by a partition between two bellows H3 and H4. The bellows I I3 contains a volatile fluid and is connected by a capillary tube II5 to a bulb IIG located adjacent the discharge side of the evaporator II. Upon an increase in temperature of the refrigerant leaving the evaporator II, the bellows H3 is expanded to move the pin I I2 upwardly to admit more refrigerant to the evaporator II. The bellows H4 is connected by a pipe III to the discharge side of the evaporator II so that the bellows I I4 is operated in accordance with variations in pressure existing within the evaporator II. Upon an increase in pressure, the bellows II4 moves the pin II2 downwardly to decrease the supply of refrigerant to the evaporator II. The temperature bellows H3 and the pressure bellows II4 operate against each other to maintain a desired temperature and pressure condition at the outlet side of the evaporator II. A

rod II8 carried by the partition between the bellows H3 and H4 is provided with a nut II9.

One end of a'compression Spring I20 engages the nut H9 and the other end thereof engages a crosshead I2l slidably mounted in guides I22. The compression spring I20 urges the rod II8 downwardly and hence the pin I I2 downwardly to assist the pressure bellows I I4. By adjusting the compression in the compression spring I20 the superheat setting of the expansion valve I9 may be adjusted at will which will vary the portion of the evaporator area required to produce the required superheat. The temperature of the evaporator is further afiected at the higher superheat values by the temperature being maintained in the fixture. Thus if the fixture temperature is 38 and the superheat setting is 20, it is obvious that the evaporator temperature must be below 18 in order to produce 20 of superheat. The amount of refrigerant in the evaporator will accordingly be controlled by the temperature of the fixture and the superheat setting of the expansion valve and hence the effective cooling area will be regulated by these conditions. By moving the crosshead I2I downwardly the spring I20 becomes more efiective to increase the superneat setting of the valve I9 which decreases the effective cooling area of the evaporator II. Conversely, upward movement of the crosshead I2I decreases thecompression in the spring I20 and lowers the superheat setting of the valve I9 to increase the effective cooling area of the evaporator II.

The crosshead I2I may be operated through a pitman I23 and a crank arm I24 by a proportioning motor I25 which may be of the typ shown and described in Patent 2,028,110 granted to D. G. Taylor on January 14, 1936. Power may be supplied to the proportioning motor I25 by means of wires I28 and I21 leading from some source of power not shown. Proportioning motor I25 may be provided with control terminals I28,

I29, and I30 for controlling the direction and ex-- tent of movement thereof.

The relative humidity responsive controller 24 may include a potentiometer resistance element I35 and a slider I36 pivoted at I31. I'he resistance element I35 and the slider I36 are connected by wires I38, I39, and I40 to the control terminals I28, I29, and I30 of the proportioning motor I25. A tension spring I4I urges the slider I38 downwardly and the slider I38 is moved upwardly by a humidity responsive device I42 upon a decrease in relative humidity. Accordingly, the slider I36 is moved with respect to the resistance element I35 upon changes in relative humidity within the fixture I0. Upon an increase in relative humidity, the slider I35 is moved downwardly to operate the proportioning motor I25 in a direction to move the crosshead I2I downwardly. Upon a decrease in relative humidity, the slider I36 moves upwardly to operate the proportioning motor I25 in a direction to move the crosshead I2I upwardly. Accordingly, as the relative humidity in the fixture I0 increases, the superheat setting of the expansion valve I9 is increased to decrease the amount of refrigerant in the evaporator II and hence decrease the effective cooling area thereof. Upon a decrease in relative humidity within the fixture I0, the superheat setting of the expansion valve I9 is decreased to increase the amount of refrigerant in evaporator II and hence to increase the efiective cooling area thereof. In other words, the effective cooling area of the evaporator II is adjusted by variations in relative humidity within the fixture I0.

Assume now that the relative humidity within the fixture I0 rises, the efieotive cooling area of the evaporator II is decreased so that the cooling action of the evaporator II is decreased. Since the displacement of the compressor I3 re mains constant and since the effective cooling area of the evaporator II is decreased, that portion of the evaporator containing refrigerant will operate at a lower temperature thereby condensing out more of the moisture in the air. In this manner the moisture content of the air is reduced to reduce the relative humidity thereof to the desired value. Conversely, upon a dei crease in relative humidity within the fixture I0,

the efiective cooling area of the evaporator II is increased and since the capacity of the compressor I3 remains constant the evaporator II will operate at a higher temperature to decrease the amount of latent heat removal and allow the relative humidity in the fixture to rise. In this manner not only is the temperature of the fixture maintained at the desired value but also the relative humidity is maintained at the desired value by controlling the proportion of latent and sensible heat removal.

Although for purposes of illustration the principles of this invention have been disclosed as applied to a single fixture arrangement, it is readily apparent that the principles of operation of this invention may equally as well be applied to a multiple fixture arrangement wherein a thermostat in each fixture is arranged in parallel for operating the compressor in conjunction with the unitary control arrangement, wherein the thermostats in the fixtures control the solenoid valves in the liquid lines leading to their respective fixtures, wherein each fixture may be provided with an adjustable superheat expansion valve, and

wherein a humidostat responsive to the relative I humidity in each fixture controls the superheat setting of its associated superheat expansion valve. With such an arrangement, exactly the same operation would be obtained in a multiple fixture arrangement as is obtained in the single fixture arrangement as illustrated in this application.

Although for purposes of illustration one form of this invention has been disclosed, other forms thereof may become apparent to those skilled in the art upon reference to this disclosure and therefore this invention is to be limited only by the scope of the appended claims and prior art.

I claim as my invention:

1. In a control system for a refrigerating apparatus having evaporator means for cooling a space in which are stored perishable articles which require high relative humidity conditions and relatively low temperature conditions for satisfactory preservation of the same and also having means for circulating refrigerant at below freezing temperatures through the evaporator means, the combination of, means responsive to variations in space temperature for controlling the circulation of refrigerant through the evaporator means at below freezing temperatures vto maintain a temperature in the space not exceeding 40' F. necessary for the satisfactory preservation of the perishable articles, means for defrosting periodically at sufliciently frequent intervals the evaporator means to prevent accumulation of frost on the evaporator means to allow the.'evaporator means to operate at the highest possible temperature in cooling the space whereby the removal of latent heat thereby is reduced to a minimum to allow relative humidity conditions to exist in the space which are too high for the satisfactory preservation of the perishable articles, and means including means responsive to the relative humidity in the space for progressively varying the effective cooling area of the evaporator means in a manner to progressively decrease the effective cooling area as the relative humidity of the space progressively increases above approximately 75%; the temperature of the efiective cooling area decreasing and the removal of latent heat increasing as the effective cooling area is decreased, whereby the relative humidity of the space is accurately maintained at the desired value necessary for the satisfactory preservation of the perishable articles.

2. The method of controlling a refrigerating apparatus having evaporator means for cooling a space in which are stored perishable articles which require. high relative humidity conditions and relatively low temperature conditions for satisfactory preservation of the same, comprising the steps of, operating the refrigerating apparatus to cause circulation of refrigerant through the evaporator means at below freezing temperatures for cooling the space to the desired relatively low temperature value necessary for the satisfactory preservation of the perishable articles and not exceeding F., defrosting periodically at sufficiently frequentintervals the evaporator means to prevent accumulation of frost on the evaporator means to allow the evaporator means to operate at the highest possible temperature in cooling the space whereby the removal of latent heat thereby is reduced to a minimum to allow relative humidity conditions to exist in the space which are too high for the satisfactory preservation of the perishable articles, and progressively decreasing the effective cooling area of the evaporator means as the relative humidity of the space increases above approximately the temperature of the effective cooling area decreasing and the removal of latent heat thereby increasing as the effective cooling area is decreased, whereby the relative humidity of l the space is accurately reduced to the desired ALWIN B. NEWTON. 

